Fix validation issues

Fix various issues found by xmllint run:
* Fix location to common.ent
* Add namespace declarations to content.ent
* Fix URLs
* replace xmlns="http://wadl:docbook.org/ns/wadl:docbook"
  which gives an "Invalid URL error" with proper
  xmlns="http://docbook.org/ns/docbook"

Command used:
xmllint --noent --noout --schema \
http://docs.rackspace.com/rackbook/wadl.xsd *.wadl

Change-Id: I1acd975e3cf1ac38be17ad2b382430ba5374a014
Merge "Fix validation issues"